Spark plug



SPARK PLUG Filed Dec. 1'7, 1928 InVsntors II. Philip -r-\/ n oer;

Their" 'Attorney F-*Hoo reous material may constructions yafter' comparatively short servicey cause Patented July 26, 1932` UNITED 's'ra'ras Am'rlsNr ori-ICE PHILIP IK. "DEvERs,

, L OF SA'UGUS, MASSACHUSETTS, AS- CORPORATION'4 0F NEW YORK rsrusuax PLUG Applicationled December 17, '1928. Serial No. 326,433.

`The present invention relates to the construction of spark plugs for internal combustion engines. It is the object of our 1nvention to provide a construction whereby a reliable and long-lived spark plug containing an insulator composed of refractory vitbe provided. y

Various attempts have been made to utilize vitreousmaterials, as for example,'fused silica and refractory glass, for the insulating member in spark plugs, but with the heretofore employed weaknesses have so frequently developed in the vitreous member that such plugs have not been suiiciently reliable to warrant their adoption on a large scale in commercial manufacture. In suchl prior plugs the vitreous insulator hasbeen constructed either with an angular shoulder to which the pressure requiredfor sealing the plug has been applied or the vitreous insulator has been made tapered in shapeand held under pressure in a tapered casing. Either construction results in strains before or during service wlfiici a1 ure of the plugs.

In accordance with otr invention we have provided an improved spark plug containing avitreous insulator having a generally elongated shape and an enlargement having convex bearing surfaces of substantially spherical configuration. Pressure is' applic to the spherical bearing surfaces whereby the enlargement is put under compression in order to produce a gas-tight seal, but shearing or bending stresses are novel features of my invention will be pointed out with greater particularity in the appended claims.

The` accompanying drawing shows several embodiments of our invention. iFigs. 1 an 2 show longitudinal sections of spark plugs containing vitreous insulators embodying our invention; Figs. 3 and 4 are longitudinal sections of modified forms of hollow insulators with a conductor mounted therein; and Figs. 5 and 6 illustrate modifications also in vertical section.

A,Referring to Figs. l and 2, the spark plug includes a casing l which is screw-threaded avoided. The.

externally at its lower end for engagement with a screwhreaded orifice in the internal combustion engine. This casing preferably consists of steel, brass, nickel or other suitable material. This casing is .screw-threaded internally at its upper end for engagement v with a retaining nut 2 whereby pressure is applied upon the vitreous insulator 3 through the soft metal gaskets 4 and 5. These gaskets land 5 preferably consist of soft copper. The lower gasket 5 may be omitted in some cases.

The insulator 3, which preferably consists of fused silica, has a bulbous enlargement 6 intermediate a shaft 7 and a shaft 8 surrounding the firing tip of the conductor 9. Theareas of this enlargement upon which `'pressure is appliedvshould approximate 'a true spherical surface, very closely in their curvature. The bearing' surfaces ofthe spheroidal" enlargement with which the gaskets 4 and 5, or other sealing means, en

gage are spaced apart leaving the intervenlng portion free. These bearing surfaces are positioned in what may be termed a median zone of the enlargement, spaced away from the shafts 7 and 8'. The silica is placed under compression in this zone without shear or tension. This spherical shape has two important functions; first, it -distributes in a most favorableway the stresses caused by d mechanical pressure and thermal expansion.

The elasticity of silica being nearly perfect. such a spherical body under pressure will adapt itself to the expansion and contraction of the external casing during temperature changes to maintain a gas-tight joint at all l times. Secondly, the spherical surfaces refleet radiant energy from the firing tip in the direction of the external casing and thus d dissipate the heat. In they absence of a structure whereby the heat derived from the firing tip is .reflected to the casing where it 1s dissipated, the transmission of heat through a silica member longitudinally would be suflicient to cause damage to external parts and the ycentral conductor. The spheroidal enlargement merges wlth the shafts 7 and 8 by a non-angular contour, as shown in the drawing, the merging surfaces preferably being reversely curved, or ofl concave curvature.

A longitudinal passage extends through the silica insulator 3 through which passes the conductor 9 havi/ng a slightly smaller diameter than the bore of the passage. As

' shown in the drawing the diameter of the conductor 9 is materially less than the diameter of the insulator 3. rPhe space between the conductor andthe walls of the passage is filled with a cement 17, for example, a mixture ofkaolin, china clay, and water glass, forming a sealing means which is inherently yieldable and will not cause the insulator to crack when the conductor 9 expands due to heating. Such a cement is commonly used in spark plug manufacture. A conductor 10 is affixed to the casing 1 and extends toward the conductor 9, being separated by a required sparking gap.

1n Fig. 1 a spacing ring 11, which may consist of copper, has been provided between the retainingy nut 2 and the silica insulator 3 in order to damp out vibrations of the insulator during service. This ring may be introduced by pouring molten metal into the space between the nut 2 and the insulator 3, a suitable plug 12 of highly refractory metal, such as powdered silica, magnesium, zirconia, or the like being provided to prevent the molten meta-l from coming into contact with the spherical surface of the insulator. in the. construction shown in Fig.

2 the members 11 and 12 have been omitted, as they are not essential in all cases.

As shown `in Fig. 3, the enlargement 6 of the insulator 3 is hollow. T he enlargement,

therefore, has two reflecting surfaces from which radiant energy is reflected and dissipated. rlhe space between the conductor 9 and the spherical enlargement 6 may be filled wholly or partly with a spark plug cement 17. lVhen it is desired to provide a hollow, spherical enlargement, it is convenient to employ tubing for the fabrication of the insulator 3, the tubing being shaped by gas pressure, applied internally while the tubing is heated to plasticity preferably within a mold of suitable form.

In the modification of our invention illustrated by Fig. 4, the shaft 7 of the silica insulator has a somewhat larger bore than in the insulators of Figs. 1 to 3 and a tube 13 of silica surrounds the conductor 9 up to the junction of the spherical enlargement 6 and thefiring tip 8. Preferably the end of the silica tube 13 is united by fusion with the shaft 7 of the insulator, as indicated at 14.

The shaft 7 of the silica insulator shown in Fig. 5 is tapered to reduce its mass at its external end ,in order to minimize the effect of mechanical vibration. ln both of the modifications of the silica insulator shown in Figs. 5 and 6 a longer shaft surrounds the firing tip than in thedcvices illustrated by the shell 1, the gasket 15 being shaped to make aline contact. Such a plug may be made gas-tight by hand.

What we cla/im as new and desire to secure by Letters Patent of the United States, is:

1. A spark plug comprising an elongated insulator of vitreous silica, said insulator having a hollow bulbous enlargement and a` hollow shaft integral therewith, a conductor extending longitudinally through said insulator, a tube of vitreous silica. extending through said shaft and said enlargement closely surrounding said conductor, a casing for said insulator, and means for making a gas-tight seal between said insulator and said casing.

2. A spark plug for internal combustion engines comprising a metal shell, elongated insulating means therein consisting essentially of vitreous silica having at one end a shaft, at the opposite end a firing tip, and intermediate thereto an enlargement having a convex spherical surface merging with said shaft and tip by concave, non-angular transition surfaces and having a longitudinal passage, aconductor of small diameter as compared with the thickness of the insulating means extending through said passage'leaving a space surrounding said conductor, aA

cement sealing the space surrounding said conductor in said passage, and means for exerting sealing pressure between said shell and the spherical surface of said enlargement.

our hands this 14th day of December, 1928. PHILIP K. DEVERS. IRVIN F. HO'OPER.

lnwitness whereof, we have hereunto setv 

